Fuel cells offer considerable promise as clean, efficient and direct sources of electric power. Recent technological advances in the design of fuel cells have developed units that may be sold in the marketplace. In addition, various economic and institutional trends are increasing the potential market value of fuel cells. This is particularly true for stationary-power applications where the inherent characteristics of the fuel cell offer distinct advantages over more conventional systems.
Although recent development efforts have successfully overcome many of the existing barriers to practical implementation of the fuel cell, a number of significant technical problems must be satisfactorily resolved. One specific problem deserving attention concerns the occurrence of thermal discontinuities within the body of the fuel cell structure and the impact of these non-uniformities on the durability and efficiency of the cell. This deviation from ideal isothermal conditions is primarily due to the fact that the oxidizing agent, such as an air stream, enters the fuel cell at a temperature that is significantly below the operating temperature of the cell. As a result, the air stream heats up as it flows through the fuel cell module. The nonDocket uniformity problems are compounded by the fact that the heat generated by the fuel cell is also nonuniform due to the variations in the composition of the fuel and air streams from the inlet to the outlet of the cell.
FIG. 1 shows the major features of a prior art fuel cell module 20. The fuel cell shown in FIG. 1 has a rectilinear planar geometry and a cross-flow orientation of the air and fuel streams. A fuel cell design having a planar geometry is desirable because the design facilitates assembling together individual fuel cell modules to provide a larger fuel cell system. The cross-flow arrangement of the air and fuel streams simplifies the design of the inlet and outlet distribution headers required to physically separate the two streams. One major disadvantage of this arrangement, however, is that it leads to a more complex and undesirable two dimensional distribution of temperature, current density, and heat generation in the fuel cell module.
The basic components of a fuel cell 20 include a core section 22 having an electrolyte layer 24 that is sandwiched between the fuel electrode 26, typically referred to as the anode, and the air electrode 28, typically referred to as the cathode. The anode 26 has a first surface 30 and a second surface 32, while the cathode 28 also has a first surface 34 and a second surface 36. The electrolyte layer 24 lies between the second surface 36 of the cathode 28 and the first surface 30 of the anode 26. The fuel cell 20 also includes an air flow distributor 36 in contact with the cathode 28 and a fuel flow distributor 38 in contact with the anode 26. The air flow distributor 36 and the fuel flow distributor 38 serve as current collectors and contain respective passages 40 and 42 which direct the flow of the two air and fuel streams over the electrode surfaces. The chemical process occurring in the fuel cell 20 consists of oxidation of the gaseous fuel. The oxidation reaction occurs on the first surface 34 of the cathode 28 where the reactive fuel species (usually H.sub.2) are oxidized to the product species (H.sub.2 O). The required source of oxygen is provided by the air stream through a mechanism which depends upon the particular type of electrolyte being utilized. When using solid oxide fuel cells, oxygen ions are generated at the surface of the cathode and migrate through the electrolyte to the anode. The electrochemical reaction generates an electric current through a connected external load and produces an accompanying quantity of heat within the fuel cell.
Fuel cells are designed to operate at the highest temperature possible because the efficiency of a typical cell increases as the operating temperature thereof increases. In most designs, the excess heat generated within the fuel cell during normal operation thereof must be continuously removed from the cell to maintain the internal temperature at an acceptable steady state level. In most designs, the waste heat is carried away primarily by the depleted air stream 44. The fuel stream also contributes to heat removal. However, the fuel stream removes significantly less heat than does the air stream because the mass flow rate of the fuel stream is lower and the inlet temperature of the fuel stream is usually higher than the air stream. As a result, temperature discontinuities and/or non-uniformities typically develop within the fuel cell. FIGS. 2A and 2B show two graphs comparing the potential magnitude of this effect for parallel flow and counterflow arrangements using a molten carbonate fuel cell. The information depicted in the graphs was presented in a report authored by T.L. Wolf and G. Wilemski, entitled "Molten Carbonate Fuel Cell Performance Model," Journal of Electrochemical Society: ELECTROCHEMICAL SCIENCE AND TECHNOLOGY, Vol. 130, No. 1, pp. 48-55, Jan. 1983. The data presented are calculated analytical results for a representative set of conditions for the simple case where both streams enter at a temperature of 800.sup.0 K. As can be seen, the spatial profiles of both cell temperature and waste heat (proportional to current density) can be very steep. Comparable variations in two dimensions would be obtained for cross-flow geometries.
The prior art fuel cell shown in FIG. 1 typically produces significant temperature discontinuities or non-uniformities within the body of the fuel cell. Significant temperature non-uniformities within fuel cells are highly undesirable for at least two Who reasons. First, large temperature gradients within the cell module can lead to excessive levels of thermal stress and premature component failure. Secondly, the temperature of the cell affects the achievable current density and the efficiency of the cell. As a result, if portions of the cell are at temperatures below the desired level, these areas may produce a potential loss in both output and efficiency compared to the ideal isothermal operating condition. In accordance with certain principles, the temperature non-uniformity condition may be at least partially alleviated by removing the waste heat as it is formed and transferring it to the air stream so as to preheat the air before it comes into contact with the active surface of the cathode portion of the cell.
There have been a number of proposed solutions to the above-identified problems. One method includes preheating the air stream. The preheating method involves utilizing a recuperator for transferring heat from the high temperature depleted air stream being discharged from the fuel cell to the fresh air stream entering the fuel cell. However, in order to insure that the temperature rise through the fuel cell is small enough the total air flow rate would have to increase substantially. This would alter the overall stoichiometry of the system and would require a corresponding increase in blower power associated with providing the higher flow rate.
Another alternative method involves embedding heat pipes in the fuel cell to provide conductive paths for transferring heat to the air stream before it enters the inlet. However, the high operating temperatures of fuel cells would require that the heat pipes include liquid metal, a rather exotic and costly design. In addition, the task of incorporating heat pipes without compromising the basic function of the fuel cell is most likely too complex to be useful.
Thus, there is a need for a fuel cell that has a simple, functional design and that solves the temperature uniformity problems discussed above. Specifically, there is a need for a fuel cell having a flow distributor that effectively transfers heat from His the active electrode surface of the fuel cell to the incoming air stream in a direction substantially away from the surface so that the temperature of the incoming air stream reaches the temperature of the active electrode surface before the air stream actually contacts the surface. As a result, planar temperature gradients in the electrode, which can lead to undesirable thermal stresses in the material, will be minimized because convective heat transfer at the surface is essentially eliminated.